In general, a flash memory device can be fabricated on a semiconductor substrate with a floating poly gate having N-type conductivity (i.e., an N-type floating gate). The N-type floating gate and the semiconductor substrate may have an insulating layer interposed therebetween. The insulating layer may electrically insulate the floating gate from the semiconductor substrate.
Electrons stored in the N-type floating gate may pass through the electric barrier of the insulating layer and may be lost through the semiconductor substrate while the flash memory device is operating. In order to reduce the loss of the stored electrons, a P-type conductivity floating poly gate (i.e., a P-type floating gate) may be used in the flash memory device. Since electrons stored in the P-type floating gate pass through the band gap of the floating gate and the electric barrier of the insulating layer and contact the semiconductor substrate, fewer electrons may be lost. On the other hand, impurity ions in the P-type floating gate may more readily diffuse into the semiconductor substrate through the insulating layer. For this reason, it may be difficult for the P-type floating gate to have a desired resistance.
Consequently, a P-type floating gate may degrade the performance of a transistor in the non-volatile memory device.